The image shows a sequence of numbers and asks observers to identify the missing number, as quickly as possible and without going through the digits one-by-one. Turns out, the task isn't so easy.

Even after multiple glances, it's almost impossible to spot the omitted number. The test isn't a matter of logic and doesn't require major detective skills to solve, but our brains often fail to pick up the mistake. Why? Because we are so used to seeing the sequence. According to a 2013 study, when our minds are focused on one thing, we fail to notice what else is around us.

In the study, conducted by psychologists Christopher Chabris and Daniel Simon, participants were shown a video of a man dressed in a gorilla suit walking through the crowd during a basketball game. When asked to count the number of passes between team members, only half of the people spotted the costumed man in the crowd.

“We have a bias to believe that we will notice all the important things,” said Chabris, a professor at Union College and co-author of “The Invisible Gorilla” study.

Apparently, when we are so focused on the task at hand, we often miss the obvious.

So, did you find the missing number right away, or did it take you a few tries? Here's a tip: If you slow down a bit and count the numbers numerically in your head, you just may find the answer. Hint: It lies somewhere between 20 and 40.